Modern hydraulic power transmissions generally include a variable displacement pump, one or more motors and controls which direct the displacement of the pump to comply with the needs of the motors. These controls keep the system pressure from exceeding maximum limits and also maintain flow at preset levels. Systems of this nature are typified in U.S. Pat. No. 3,508,847. A further advancement of this type of system are those generally referred to as "load responsive" which permits the variable displacement pump to maintain only that flow and pressure level necessary to move a particular load. This type of system is typified in U.S. Pat. Nos. 3,401,521 and 4,037,410. One of the drawbacks to all of these above-mentioned systems is that the pressure and flow controls are designed to function within a set oil temperature range and quite often the system is quite cold, as would be the situation when a tractor is left outside in the winter. Before functioning properly, the oil reservoirs must be warmed-up, which is quite difficult with closed center systems of this type.
The present invention solves this initial warm-up problem by providing a valve and a control system which circulates sufficient oil through the pump and system to warm-up the oil while not allowing the pump load to exceed the idle torque output of the engine and stall the machine. One prior art method of dealing with thick cold oil is shown in U.S. Pat. No. 3,637,327 wherein the tension on the pressure compensating spool of the variable displacement pump is varied by use of a bimetallic Bellville spring. The system oil is exposed to the Bellville spring causing the spring force on the pressure compensating spool to vary in accordance with the temperature of the oil so as to provide a temperature compensated system which maintains a fairly constant pressure level regardless of the oil temperature.
The present invention deals with this increased viscosity problem automatically with a warm-up valve which in its cold condition directly connects pump discharge flow to the pump compensator while allowing a limited percentage of the pump's flow capacity to flow back to reservoir at a sufficient pressure to warm-up the system. By allowing a limited capacity of oil to be pumped back to the reservoir, the torque output of the engine in idle or fast-idle speeds will not be overcome by the pump which would stall the engine. The valve is also thermostatically controlled so that when the oil temperature reaches a certain operating temperature level, the direct pressure from the pump is cut-off to the pump compensator thereby causing the pump to return to a low pressure standby condition.
It is therefore the principal object of the present invention to provide a warm-up valve in variable displacement systems which automatically circulates to reservoir a limited amount of oil in an idle condition to warm the reservoir while not exceeding the torque output of the engine in idle.
Another object of the present invention is to provide a warm-up valve in a load responsive system which automatically sets the pump compensator at a flow and pressure rate sufficient to warm the oil in the reservoir while not exceeding the torque output of the engine.
Another object of the present invention is to provide a circulating warm-up circuit which can be manually overridden by actuating one of the directional control valves of the system.
A further object of the present invention is to provide a warm-up circuit which at cranking RPM circulates flow to reservoir at a minimum pressure build-up.